This invention is related to the foaming of a medium which is polymerizable and/or crosslinkable by free radical initiation. More particularly, it comprehends the crosslinking of unsaturated polyester resins using t-alkylhydrazinium salt blowing agents with transition metal salt promoters.
The foaming and curing of unsaturated polyester resins are well known in the prior art. U.S. Pat. No. 3,920,589 discloses the foaming and curing of an unsaturated polyester resin using a peroxide curing agent, a transition metal salt promoter, hydrazine or a sulfonyl hydrazide blowing agent, and a halogen redox compound. U.S. Pat. No. 3,920,590 discloses the foaming and curing of an unsaturated polyester resin using a peroxide curing agent, a cobalt promoter, and hydrazine or oxybis (benzesulfonyl) hydrazide blowing agent, wherein both the peroxide and cobalt are present in high concentration levels. U.S. Pat. No. 3,920,591 discloses the foaming and curing of an unsaturated polyester resin using a peroxide curing agent, organo-metallic salt promoters of transition metals, hydrazine or a sulfonyl hydrazide blowing agent, and an aliphatic amine redox compound. Published European patent application No. 0048050 discloses the foaming and curing of an unsaturated polyester resin using a peroxide curing agent, an organometallic salt promoter, and a t-alkyl hydrazine blowing agent. None of the above-mentioned prior art teaches the present invention.